Tissue modification

ABSTRACT

A tissue modification apparatus includes a first and a second guide, a first and a second set of mounts, and a first and a second actuator. The first guide defines a first axis, and the second guide defines a second axis intersecting the first axis. The mounts of the first set of mounts are movable relative to one another along the first axis, and the mounts of the second set of mounts are movable relative to one another along the second axis. The first actuator and the second actuator are each settable to a stress load, with the first actuator and the second actuator movable, respectively, along the first axis and the second axis to transmit each respective stress load to a piece of tissue mechanically coupled to the first and second set of mounts.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/539,675, filed on Sep. 27, 2011, which is incorporated byreference herein.

TECHNICAL FIELD

The following disclosure relates to tissue modification and, moreparticularly, to modification of biological tissue for implantation in amammal.

BACKGROUND

Heart valve surgery can be used to repair or replace diseased heartvalves. For example, heart valve replacement may be indicated when thereis a narrowing of the native heart valve, commonly referred to asstenosis, or when the native valve leaks or regurgitates. The repair orreplacement of diseased heart valves can include, for example, theintroduction of a prosthetic heart valve that includes biological tissueheterologous to the patient (e.g., a heterograft or xenograft).

Biological tissue can have mechanical properties that vary within asingle donor and/or from among several donors of the same species. Forexample, biological tissue from a single donor can have non-uniformthickness, and the average thickness of biological tissue can vary fromone donor to another. The variation in mechanical properties ofbiological tissue used in replacement heart valves can impact theperformance and/or durability of a replacement heart valve implanted ina patient.

SUMMARY

Tissue modification changes one or more mechanical properties ofbiological tissue used for implantation in a mammal.

In one aspect, a tissue modification apparatus includes a first and asecond guide, a first and a second set of mounts, and a first and asecond actuator. The first guide defines a first axis, and the secondguide defines a second axis intersecting the first axis. The mounts ofthe first set of mounts are movable relative to one another along thefirst axis, and the mounts of the second set of mounts are movablerelative to one another along the second axis. The first actuator andthe second actuator are each settable to a stress load, with the firstactuator and the second actuator movable, respectively, along the firstaxis and the second axis to transmit each respective stress load to apiece of tissue mechanically coupled to the first and second set ofmounts.

In some embodiments, the second axis is substantially perpendicular tothe first axis. In certain embodiments, the second axis intersects thefirst axis in a plane substantially parallel to a substantially planarsurface of the piece of tissue mechanically coupled to the first andsecond set of mounts.

In some embodiments, at least a portion of the first guide ismechanically coupled to at least a portion of the second guide. Forexample, the first guide can be mechanically coupled to the second guideat the intersection of the first axis and the second axis.

In certain embodiments, a plurality of first guides is substantiallyparallel to or coaxial with the first axis and a plurality of secondguides is substantially parallel to or coaxial with the second axis. Forexample, the plurality of first guides can include a first pair of rodssubstantially parallel to one another and the plurality of second guidecan include a second pair of rods substantially parallel to one another.Additionally or alternatively, at least one of the first pair of rods isdisposed along the first axis and at least one of the second pair ofrods is disposed along the second axis. Each rod of the first and secondpair of rods can have an outer diameter great than or equal to about0.75 mm and less than or equal to about 13 mm. For example, each rod ofthe first and second pair of rods is stainless steel with an outerdiameter of about 3.2 mm.

In some embodiments, the first pair of rods are substantiallyperpendicular to the second pair of rods to form a substantiallycruciform frame with an aperture defined by the first pair of rods andthe second pair of rods and portions of each of the first and secondpair of rods extending away from the aperture. For example, the firstactuator and the second actuator are disposed on the respective portionsof the first and second pair of rods extending away from the aperture.Additionally or alternatively, the first set of mounts and the secondset of mounts are disposed on the respective portions of the first andsecond pair of rods extending away from the aperture.

In certain embodiments, the first guide and the second guide eachinclude at least one passivated surface. Additionally or alternatively,the first guide and the second guide can each include at least onepolished surface.

In some embodiments, the first guide and the second guide each includeat least one collar stationary relative to the respective first andsecond guide. The at least one collar can be disposed along therespective first and second axes, and the at least one collar can limitmovement of the respective first and second set of mounts along therespective first and second axes.

In certain embodiments, each mount of the first set of mounts is largerthan each of the second set of mounts along axes perpendicular to therespective first and second axes. For example, each mount of the firstset of mounts can extend about 100 mm to about 150 mm in a directionperpendicular to the first axis and each mount of the second set ofmounts can extend about 50 mm to about 90 mm in a directionperpendicular to the second axis. In some embodiments, each mount of thefirst set of mounts and the second set of mounts is substantiallycylindrical with an outer diameter of about 6 mm to about 25 mm.

In some embodiments, each mount of the first and second set of mountsincludes mitered end portions. For example, the mitered end portions ofeach mount of the first set of mounts can be complementary to themitered end portions of each mount of the second set of mounts.

In certain embodiments, the first and second actuators are each movablealong the respective first and second axes to transmit each respectiveset stress load substantially simultaneously to a piece of tissue.

In some embodiments, the first and second actuators each comprise atleast one spring. For example, the at least one spring of each first andsecond actuator can be compressible to set the respective stress loadand expandable transmit the respective stress load to the piece oftissue mechanically coupled to the first and second set of mounts. Theat least one spring of the first actuator and the at least one spring ofthe second actuator can have substantially similar spring constants.Additionally or alternatively, the at least one spring of the firstactuator and the at least one spring of the second actuator can havespring constants of about 5 N/m to about 50 N/m. Additionally oralternatively, the at least one spring of the first actuator and the atleast one spring of the second actuator can each be compressible fromrespective equilibrium positions by a distance of about 5 mm to about 35mm.

In certain embodiments, each mount of the first and second set of mountsincludes at least one surface in slidable contact with the respectivefirst and second guides along the respective first and second axes. Forexample, the at least one surface of each of the first and second setsof mounts in slidable contact with the respective first and secondguides is polytetrafluoroethylene.

In some embodiments, each mount of the first and second set of mountscomprises a bearing in rolling contact with a respective first andsecond guide. For example, each bearing can be polyoxymethylene orpolytetrafluoroethylene. Additionally or alternatively, each bearing canbe a flange bearing.

In certain embodiments, each mount of the first and second set of mountscomprises a plurality of hooks extending away from each respectivemount. For example, each of the plurality of hooks can be substantiallyaxially aligned with one another along each respective mount.Additionally or alternatively, the plurality of hooks can besubstantially evenly spaced along the axis.

In another aspect, a tissue modification system includes a tissuestretcher and a jig. The tissue stretcher includes a first guide and asecond guide, a first set of mounts and a second set of mounts, a firstactuator and a second actuator. The first guide defines a first axis,and the second guide defines a second axis intersecting the first axis.The mounts of first set of mounts are movable relative to one anotheralong the first axis, and the mounts of the second set of mounts aremovable relative to one another along the second axis. The firstactuator and the second actuator are mechanically coupled, respectively,to the first and second set of mounts. The jig includes a base, asupport extending from the base, and a plurality of locking members. Thefirst guide and/or the second guide are mechanically couplable to thebase such that the first and second guides are stationary relative tothe base. The plurality of locking members are mechanically couplable tothe base and movable with respect to the first and second actuators toallow movement of the first and second actuators along the respectivefirst and second axes to transmit a respective stress load to a piece oftissue mechanically coupled to the first and second set of mounts.

In some embodiments, the first and second actuators are movable relativeto the base to set respective stress loads and the plurality of lockingmembers are movable to hold the first and second actuators at therespective set stress loads.

In certain embodiments, the support and the locking members aresimultaneously mechanically decouplable from the tissue stretcher.

In some embodiments, the first guide includes a first pair of rodssubstantially parallel to one another and the second guide includes asecond pair of rods substantially parallel to one another. For example,the first pair of rods can be substantially perpendicular to the secondpair of rods to form a substantially cruciform frame with an aperturedefined by the first pair of rods and the second pair of rods andportions of each of the first and second pair of rods extending awayfrom the aperture. At least a portion of the support can be engageablewith the aperture to hold the first and second pair of rods stationaryrelative to the base. Additionally or alternatively, the first actuatorand the second actuator can be disposed on the respective portions ofthe first and second pair of rods extending away from the aperture.Additionally or alternatively, the first set of mounts and the secondset of mounts can be disposed on the respective portions of the firstand second pair of rods extending away from the aperture.

In another aspect, a tissue modification method includes moving a firstpair of mounts toward one another along a first axis to set a firstload, moving a second pair of mounts toward one another along a secondaxis, intersecting the first axis, to set a second load, mechanicallycoupling a substantially planar sheet of tissue to the first pair ofmounts and to the second pair of mounts, moving the first pair of mountsaway from one another along the first axis to apply the set first loadto the substantially planar sheet of tissue, and moving the second pairof mounts away from one another along the second axis to apply the setsecond load to the substantially planar sheet of tissue.

In some embodiments, the first axis is substantially perpendicular tothe second axis. In certain embodiments, the first stress load issubstantially equal to the second stress load. In some embodiments, thefirst stress load and the second stress load are applied parallel to thesubstantially planar surface of the tissue. In certain embodiments, thefirst and second set stress loads are applied substantiallysimultaneously.

In certain embodiments, moving the first and second pair of mountstoward one another along the respective first and second axes includescompressing one or more springs mechanically coupled to the first andsecond pair of mounts. Additionally or alternatively, moving the firstand second pair of mounts away from one another along the respectivefirst and second axes can include at least partially releasing thecompression of the one or more springs mechanically coupled to the firstand second pair of mounts.

In another aspect, a tissue modification method includes setting a firstand a second stress load to apply to a piece of tissue, applying thefirst stress load to the piece of tissue along a first axis, andapplying the second stress load to the piece of tissue along a secondaxis substantially perpendicular to the first axis, wherein the firstand second stress loads are applied to the tissue substantiallysimultaneously.

In certain embodiments, setting the first and second stress loadsincludes compressing respective first and second springs. Additionallyor alternatively, applying the first and second stress loads includes atleast partially releasing the respective compressed first and secondsprings.

In some embodiments, the first stress load and the second stress loadare substantially equal.

In certain embodiments, the piece of tissue is a substantially planarsheet and the first and second axes are parallel to the plane defined bythe substantially planar sheet. Additionally or alternatively, the pieceof tissue includes biological tissue. For example, the piece of tissuecan be one of bovine pericardium, equine pericardium, and porcinepericardium.

In some embodiments, the piece of tissue is exposed to a glutaraldehydesolution. Additionally or alternatively, the piece of tissue can beexposed to the glutaraldehyde solution during at least a portion of theexposure of the piece of tissue to the first and second stress loads.For example, the piece of tissue can be exposed to glutaraldehydesolution for about one day to about two weeks. The first and secondstress loads can be each applied to the piece of tissue for about 30minutes to about 120 minutes.

In another aspect, a tissue modification method includes arranging asubstantially planar patch of pericardial tissue in a stationaryposition relative to a base, and moving a shaver relative to thesubstantially planar patch of pericardial tissue to remove tissue alongat least a portion of a substantially planar surface of thesubstantially planar patch of pericardial tissue.

In some embodiments, the substantially planar patch of pericardialtissue has a first substantially planar surface rougher than a secondsubstantially planar surface. The shaver can remove at least a portionof the first substantially planar surface.

In certain embodiments, the substantially planar patch of pericardialtissue is bovine pericardium, equine pericardium, or porcinepericardium.

In some embodiments, vacuum pressure is applied to the substantiallyplanar patch of pericardial tissue such that the substantially planarpatch of pericardial tissue is drawn toward the base. For example, thevacuum pressure is applied to the substantially planar patch of tissueby drawing air through a plurality of orifices defined by the base.Additionally or alternatively, a saline solution can be applied to thesubstantially planar piece of tissue. For example, the saline solutionand the vacuum pressure can be simultaneously applied to thesubstantially planar piece of tissue.

In certain embodiments, the substantially planar piece of tissue isfixed and the shaver is moved relative to the stationary piece of tissueduring or after fixing the substantially planar piece of tissue.

In another aspect, a tissue modification method includes forming asubstantially planar leaflet from a piece of pericardial tissue,arranging the substantially planar leaflet in a stationary positionrelative to a base, and removing tissue from at least a portion of asubstantially planar surface of the substantially planar leaflet. Thesubstantially planar leaflet includes a coaptation portion, an arcuateedge substantially opposite the coaptation portion, the arcuate edgehaving a first end and a second end, and a belly extending from thearcuate edge to an axis defined by the first and second ends of thearcuate edge.

In certain embodiments, removing at least a portion of the substantiallyplanar surface of the substantially planar leaflet includes removingtissue from the belly of the leaflet. Additionally or alternatively,removing at least a portion of the substantially planar surface of thesubstantially planar leaflet includes moving a laser (e.g., afemtosecond laser) over a portion of the stationary, substantiallyplanar leaflet.

In another aspect, a method of prosthetic heart valve preparationincludes storing a prosthetic heart valve in a first solution, theprosthetic heart valve comprising a biological tissue isotonic to thefirst solution, exposing the biological tissue of the prosthetic heartvalve to a second solution hypertonic to the biological tissue such thatwater flows out of the biological tissue, and moving a sheath distallyover the prosthetic heart valve to contract the prosthetic heart valvefor intraluminal delivery to a mammalian heart.

In some embodiments, the second solution is about 75 percent to about 80percent water. In certain embodiments, the tonicity of the secondsolution is about 339 mOsm/L to about 12.3 Osm/L. Additionally oralternatively, the biological tissue of the prosthetic heart valve canbe exposed to the second solution for about 30 seconds to about 15minutes. Additionally or alternatively, the first solution and thesecond solution can be at substantially the same temperature.

In certain embodiments, the biological tissue is sterilized (e.g., in aterminal sterilization procedure) after the biological tissue of theprosthetic heart valve is exposed to the second solution hypertonic tothe biological tissue. For example, sterilizing the biological tissuecan include exposing the biological tissue to electron beam (e-beam)radiation and/or exposing the biological tissue to ethylene oxide (EtO).

In another aspect, a method of prosthetic heart valve preparationincludes exposing a prosthetic heart valve in a first solution, theprosthetic heart valve comprising a biological tissue isotonic to thefirst solution, exposing the biological tissue of the prosthetic heartvalve to a second solution including alcohol or ethylene glycol suchthat water flows out of the biological tissue, and moving a sheathdistally over the prosthetic heart valve to contract the prostheticheart valve for intraluminal delivery to a mammalian heart. In someembodiments, exposing the biological tissue of the prosthetic heartvalve to the second solution includes exposing the biological tissue toa series of graded alcohol solutions or a series of graded ethyleneglycol solutions. For example, the series of graded alcohol or ethyleneglycol solutions includes about 50%, about 60%, about 70%, about 90%,about 95%, and about 100% alcohol or ethylene glycol solutions.

In some embodiments, the biological tissue of the prosthetic heart valveis exposed to the second solution for about 30 seconds to about 15minutes. Additionally or alternatively, the first solution and thesecond solution can be at substantially the same temperature.

In certain embodiments, the biological tissue is sterilized (e.g., in aterminal sterilization procedure) after the biological tissue of theprosthetic heart valve is exposed to the second solution includingalcohol or ethylene glycol. For example, sterilizing the biologicaltissue can include exposing the biological tissue to electron beam(e-beam) radiation and/or exposing the biological tissue to ethyleneoxide (EtO).

In still another aspect, a method of prosthetic heart valve preparationincludes exposing a biological tissue to a first solution, exposing thebiological tissue to a second solution such that water flows out of thebiological tissue, and applying biaxial stress loads to the biologicaltissue. The biological tissue is isotonic to the first solution. Thesecond solution includes alcohol or ethylene glycol. The biaxial stressload can be applied to the biological tissue after the biological tissueis exposed to the second solution.

Embodiments can include one or more of the following advantages.

In some embodiments, stress loads to be transmitted to tissue can be setprior to being applied to a respective first axis and a second axis.Setting the stress loads in this manner can allow biaxial stress to berepeatably and reliably applied to multiple, different pieces of tissue.This repeatable and reliable application of stress load can improve theamount of tissue that can be processed, as compared to methods thatrequire more manual manipulation by an operator.

Moreover, setting the stress load in these embodiments results in a setstress to the tissue and variable amount of strain applied to pieces oftissue. For example, in these embodiments, pieces of tissue havingdifferent stress-strain characteristics will be stretched differentdistances under the same set stress load. Setting the stress load andallowing the resulting strain in the tissue to vary can improve theuniformity of the mechanical properties (e.g., stiffness along a firstaxis and a second axis) across several pieces of tissue. This improveduniformity can facilitate tissue matching for leaflets used in aprosthetic heart valve, where matching mechanical properties of thetissue used for the leaflets can improve the load distribution over theleaflets and, thus, improve hemodynamic performance and/or improve thedurability of the prosthetic heart valve.

In certain embodiments, set stress loads are applied substantiallysimultaneously along the respective first and second axes. As comparedto biaxial stress load that is not applied substantially simultaneously,the application of substantially simultaneous biaxial stress load totissue to apply a substantially simultaneous stress to the tissue canimprove the mechanical properties of the tissue. For example, exposingtissue to substantially simultaneous biaxial stress loading can improvethe similarity between stiffness of the tissue along the first andsecond axes. In a prosthetic heart valve including leaflets made fromtissue, the improvements in the similarity in stiffness characteristicsalong the first and second axes of the tissue can result in improvedload distribution and/or durability in the valve.

In some embodiments, a shaver is moved relative to a planar patch ofpericardial tissue to remove tissue along at least a portion of thesubstantially planar surface of the substantially planar patch ofpericardial tissue. This removal of tissue can improve the uniformity ofthickness of the planar patch of tissue. In instances in which thetissue or a portion of the tissue is part of an intraluminally deliveredprosthetic heart valve, the improved uniformity of thickness of thetissue can result in reduced forces associated with sheathing theprosthetic heart valve for intraluminal delivery.

In certain embodiments, the substantially planar patch of pericardialtissue has a first substantially planar surface rougher than a secondsubstantially planar surface and the shaver is moved relative to theplanar patch of pericardial tissue to remove tissue from the roughersurface. Removal of tissue from the rougher substantially planar surfaceof the pericardial tissue can decrease the thickness profile of thetissue while substantially maintaining the overall mechanical propertiesof the tissue.

In certain embodiments, a laser is moved relative to a substantiallyplanar leaflet to remove tissue from at least a portion of asubstantially planar surface of the substantially planar leaflet. Byselectively removing material from portions of the substantially planarleaflet (e.g., the belly of the leaflet), the laser can be used toachieve a local reduction in thickness of the tissue. This localreduction in the thickness of the tissue can reduce the forcesassociated with sheathing a prosethetic heart valve for intraluminaldelivery while maintaining the thickness of the leaflet in other areasto facilitate attachment and/or to improve load distribution.

In certain embodiments, biological tissue of a prosthetic heart valve isexposed to a solution hypertonic to the biological tissue such thatwater flows out of the biological tissue. In some embodiments,biological tissue of a prosthetic heart valve is exposed to a solutionincluding alcohol or ethylene glycol such that water flows out of thebiological tissue. Such removal of water from the biological tissuereduces one or more dimensions of the biological tissue such that forcesassociated with sheathing the prosthetic heart valve are reduced.Additionally or alternatively, such reduction of one or more dimensionsof the biological tissue can facilitate reduction of the overall outerdiameter of the inraluminal delivery system for the prosthetic heartvalve.

In certain embodiments, the tissue can be dehydrated (e.g., by exposureto a hypertonic solution, alcohol, or ethylene glycol) during the finalmanufacturing steps of a prosthetic heart valve such that the tissueundergoes terminal sterilization in a dehydrated state. As compared totissue in a hydrated state, the tissue in a dehydrated state can betterwithstand exposure to forms of terminal sterilization (e.g., e-beam andethylene oxide sterilization) that tend to damage and/or alterbiological tissue in a hydrated state. Additionally or alternatively,the dehydration of the biological tissue can facilitate sterilizationand storage of the prosthetic heart valve in a sheathed or unsheathedposition. This can, for example, facilitate the sterilization of theentire prosthetic heart valve assembly (including the valve, valvedelivery system, and handle, etc.) in a single package, without theneed, for example, for storing the valve in liquid.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other aspects,features, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a partially exploded isometric view of a tissue modificationsystem.

FIG. 2 is a top view of a tissue stretcher of the tissue modificationsystem of FIG. 1.

FIG. 3 is a top view of a jig of the tissue modification system of FIG.1.

FIG. 4 is a top view of the tissue modification system of FIG. 1 in acompressed state.

FIG. 5 is a top view of the tissue modification system of FIG. 1 in acompressed state and mounted with tissue.

FIG. 6 is a top view of the tissue stretcher of FIG. 1 in an expandedstate and mounted with tissue.

FIG. 7 is a side view of a mount of a tissue stretcher.

FIG. 8 is a top view of a tissue modification system mounted withtissue.

FIG. 9 is a cross-sectional view of the tissue modification system ofFIG. 8 taken along the line A-A in FIG. 8.

FIG. 10 is a top view of a tissue modification system mounted with atissue leaflet.

FIGS. 11A-D are schematic representations of the process of sheathing aprosthetic heart valve including biological tissue.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIGS. 1-6, a tissue modification system 1 includes a tissuestretcher 10 and a jig 50. In use, as described in further detail below,the tissue stretcher 10 is mechanically couplable to the jig 50 suchthat at least a portion of the tissue stretcher 10 remains fixed inplace relative to the jig 50 and a substantially planar piece of tissue70 can be mounted on the tissue stretcher 10. As also described infurther detail below, the tissue stretcher 10 can be decoupled from thejig 50 such that at least a portion of the tissue stretcher 10 expandsto stretch the tissue 70 biaxially.

Referring now to FIGS. 1-2, the tissue stretcher 10 includes firstmounts 26 a,b disposed along substantially parallel first guides 12 a,band second mounts 28 a,b disposed along substantially parallel secondguides 16 a,b. First actuators 32 a,b,c,d are disposed on the firstguides 12 a,b and second actuators 34 a,b,c,d are disposed on the secondguides 16 a,b. In use, the first actuators 32 a,b,c,d can move the firstmounts 26 a,b away from one another along a first axis 14 defined by oneof the first guides 12 a,b, and the second actuators 34 a,b,c,d can movethe second mounts 28 a,b away from one another along a second axis 18defined by one of the second guides 16 a,b. The relative movement of thefirst mounts 26 a,b away from one another along the first axis 14 andthe relative movement of the second mounts 28 a,b away from one anotheralong the second axis 18 results in the application of a biaxial stressload to tissue (e.g., the tissue 70 in FIGS. 5-6) mounted on the tissuestretcher 10.

As used herein, “stress load” is the force (measured, e.g., in N)applied to the tissue. For example, in some embodiments in which thefirst actuators 32 a,b,c,d and the second actuators 34 a,b,c,d aresprings, the stress load is proportional to the spring constant and thedisplacement of the spring from its equilibrium position. In theseembodiments, the stress load can be mathematically expressed by Hooke'slaw (F=−k*x, where F is the stress load, k is the spring constant, and xis the displacement of the spring's end from it equilibrium position).As used herein, “stress” is the force (i.e., stress load) per unit areaof the tissue and can be measured, for example, in N/mm². Accordingly,the stress applied to the tissue is a function of the stress load andthe cross-sectional area of the tissue. For example, for a given stressload, the stress applied to the tissue may vary with the thickness ofthe tissue.

The first guides 12 a,b and the second guides 16 a,b are coupled (e.g.,welded, interference fit) to one another at connection regions 24defined by the intersection of the first guides 12 a,b with the secondguides 16 a,b (e.g., the intersection of the first axis 14 and thesecond axis 18). The mechanical coupling between the first guides 12 a,band the second guides 16 a,b can facilitate the application of biaxialstress load to a piece of the tissue 70 (FIGS. 5-6) mounted on thetissue stretcher 10. In some embodiments, the connection regions 24 arethe point of origin of the respective actuation forces transmitted bythe respective force actuators 32 a,b,c,d and 34 a,b,c,d to therespective first mounts 26 a,b and second mounts 28 a,b.

In some embodiments, the first and second guides 12 a,b, 16 a,b includeat least one polished surface and/or at least one passivated surface.The polished surface can, for example, reduce friction associated withthe movement of the respective first and second mounts 26 a,b, 28 a,balong the respective first and second guides 12 a,b, 16 a,b. Thepassivated surface can, for example, improve resistance of corrosion ofthe first and second guides 12 a,b, 16 a,b that could otherwise occurthrough exposure of the first and second guides 12 a,b, 16 a,b to salineand/or glutaraldehyde solutions.

The first guides 12 a,b are substantially perpendicular to the secondguides 16 a,b in a plane substantially parallel to a substantiallyplanar surface of the tissue 70 (FIGS. 5-s 6) mechanically coupled tothe first and second set of mounts 26 a,b, 28 a,b. This substantiallyperpendicular orientation of the first guides 12 a,b to the secondguides 16 a,b forms a cruciform frame 20 with an aperture 22 defined byat least a portion of the first and second guides 12 a,b and 16 a,b. Atleast a portion of each of the first guides 12 a,b and the second guides16 a,b extend away from the aperture. As described in further detailbelow, the first actuators 32 a,b,c,d and the first set of mounts 26 a,bare disposed on the portions of the first guides 12 a,b extending awayfrom the aperture 22. Analogously, the second actuators 34 a,b,c,d andthe second set of mounts 28 a,b are disposed on the portions of thesecond guides 16 a,b extending away from the aperture 22. As alsodescribed in further detail below, the aperture 22 is releasablyengageable with at least a portion of the jig 50 such that the cruciformframe 20 remains fixed relative to the jig 50 to facilitate movement ofthe first and second set of mounts 26 a,b, 28 a,b as the respectivestress loads are set.

Each of the first guides 12 a,b and the second guides 16 a,b can be arod having an outer diameter greater than or equal to about 0.75 mm andless than or equal to about 13 mm. For example, each of the first guides12 a,b and the second guides 16 a,b can be a stainless steel rod with anouter diameter of about 3.2 mm. The use of such a rod for the firstguides 12 a,b and the second guides 16 a,b can facilitate sizing of thetissue stretcher 10 for manual manipulation by an operator whileproviding sufficient rigidity for the application of loads to the tissueto be modified.

A collar 36 can be disposed on one or more of the first guides 12 a,band the second guides 16 a,b. The collar 36 can be coupled (e.g.,welded, interference fit) in a stationary position relative to one ormore of the first guides 12 a,b and the second guides 16 a,b to limitthe range of motion of the respective first and second set of mounts 26a,b, 28 a,b. For example, the collar 36 can be coupled to an end portionof the respective first and second guides 12 a,b, 16 a,b.

Each of the first and second mounts 26 a,b, 28 a,b is substantiallycylindrical (e.g., a right circular cylinder) with a substantiallyuniform cross-sectional area along the length of the cylinder. Forexample, each mount 26 a,b, 28 a,b can have an outer diameter of about 6mm to about 25 mm (e.g., about 13 mm). Additionally or alternatively,each mount 26 a,b, 28 a,b can have a length of about 50 mm to about 150mm (e.g., about 75 mm, about 125 mm). The length of each of the firstmounts 26 a,b is perpendicular to the first guides 12 a,b, and thelength of each of the second mounts 28 a,b is perpendicular to thesecond guides 16 a,b.

Each mount 26 a,b, 28 a,b includes a plurality of hooks 40 extendingaway from an outer surface of each respective mount 26 a,b, 28 a,b. Forexample, each hook 40 can extend about 1.5 mm from the surface of eachrespective mount 26 a,b, 28 a,b. This hook size can facilitate, forexample, secure attachment to the tissue 70 (FIGS. 5-6) while providinga low profile to reduce the likelihood of unintended snagging duringhandling. The hooks 40 can be substantially axially aligned with oneanother along the length of the respective first and second mounts 26a,b, 28 a,b. Additionally or alternatively, the plurality of hooks 40can be substantially evenly spaced along the length of each respectivemount 26 a,b, 28 a,b. This axial alignment and substantially uniformspacing of the hooks 40 can result, for example, in substantiallyuniform application of stress loads to the tissue 70. In someembodiments, each hook 40 formed from a cut mandrel.

Each of the first and second mounts 26 a,b, 28 a,b has mitered endportions 38 such that the respective mitered end portions 38 of each ofthe first mounts 26 a,b is complementary to the mitered end portions 38of each of the second mounts 28 a,b. These mitered end portions 38 canfacilitate setting the biaxial loads to be applied by the tissuestretcher 10 by, for example, allowing the first mounts 26 a,b and thesecond mounts 28 a,b to be moved into close proximity with one another(e.g., by abutting the complementary mounts) during loading.

Additionally or alternatively, the length of each of the first mounts 26a,b can be greater than the length of each of second mounts 28 a,b. Forexample, each of the first mounts 26 a,b can have a length of about 100mm to about 150 mm (e.g., about 125 mm), and the second mounts 28 a,bcan have a length of about 50 mm to about 90 mm (e.g., about 75 mm).This relative difference in the lengths of the first and second mounts26 a,b, 28 a,b can also facilitate setting the biaxial loads to beapplied by the tissue stretcher 10 by, for example, allowing the firstmounts 26 a,b and the second mounts 28 a,b to be moved into closeproximity with one another during loading.

Each mount 26 a,b, 28 a,b includes at least one surface in slidablecontact with the respective first and second guides 12 a,b, 16 a,b suchthat the first mounts 26 a,b are slidable relative to one another alongthe first axis 14 and the second mounts 28 a,b are slidable relative toone another along the second axis 18. In some embodiments, the at leastone surface of the respective mounts 26 a,b, 28 a,b in slidable contactwith the respective first and second guides 12 a,b, 16 a,b ispolytetrafluoroethylene (PTFE), which can result in low friction betweenthe first and second mounts 26 a,b, 28 a,b and the respective first andsecond guides 12 a,b, 16 a,b. Such low friction can be useful for theefficient and consistently uniform transmission of stress loads from thefirst and second actuators 32 a,b,c,d, 34 a,b,c,d to the respectivemounts 26 a,b, 28 a,b and, thus, ultimately to the tissue 70 (FIGS. 5-6)to be modified.

The first and second actuators 32 a,b,c,d, 34 a,b,c,d are each movablerespectively along first and second guides 12 a,b, 16 a,b. For example,each of the first and second actuators 32 a,b,c,d, 34 a,b,c,d can bemoved toward the aperture 22 to a fixed position relative to theaperture 22, with this fixed position corresponding to a set (e.g.,predetermined) stress load to be applied to the tissue 70 (FIGS. 5-6).As described in further detail below, the first and second actuators 32a,b,c,d, 34 a,b,c,d can be released from this fixed position such thatthe respective set stress loads are applied to the tissue 70 mounted tothe first and second mounts 26 a,b, 28 a,b. In some embodiments, therelative position of the first and second actuators 32 a,b,c,d, 34a,b,c,d with respect to the aperture 22 is proportional to therespective set stress load that will be applied by each actuator to thetissue 70 coupled to the first and second mounts 26 a,b, 28 a,b.

In some embodiments, the first and second actuators 32 a,b,c,d, 34a,b,c,d are substantially simultaneously movable along the respectivefirst and second guides 12 a,b, 16,a,b to transmit biaxial stress loadsto the tissue 70 (FIGS. 5-6). For example, each of the first and secondactuators 32 a,b,c,d, 34 a,b,c,d can be a spring such that the stressload is set through compression of each spring relative to theequilibrium point of the spring. Each compressed spring can be allowedto expand toward the equilibrium position to move the respective firstmounts 26 a,b away from one another and/or move the respective secondmounts 28 a,b away from one another and, thus, apply respective stressloads to the tissue (FIGS. 5-6). Each spring of the first and secondactuators 32 a,b,c,d, 34 a,b,c,d can have a spring constant of about 5N/m to about 50 N/m. Additionally or alternatively, springs of the firstand second actuators 32 a,b,c,d, 34 a,b,c,d are each compressible fromrespective equilibrium positions by a distance of about 5 mm to about 35mm.

In some embodiments, the first and second actuators 32 a,b,c,d, 34a,b,c,d are springs having substantially similar spring constants suchthat the stress loads applied along the first axis 14 and the secondaxis 18 are substantially equal. In certain embodiments, the firstactuators 32 a,b,c,d have a spring constant greater than the springconstant of the second actuators 34 a,b,c,d such that a larger stressloading can be applied along the first axis 14 than along the secondaxis 18.

Referring now to FIGS. 1, 3, and 4, the jig 50 includes a support 54extending from a substantially planar base 52. The base 52 defines aplurality of locking apertures 58. Locking members 56 (e.g., pins) aremechanically couplable to the base 52 (e.g., by interference fit withone or more of the locking apertures 58) and extend away from the base52 on the same side of the base 52 as the support 54. It should beappreciated that the locking members 56 can be disposed in any of thelocking apertures 58, as required to set a desired stress load byrestricting axial movement of the first and second actuators 32 a,b,c,d,34 a,b,c,d. It should be further appreciated that moving the lockingmembers 56 away from the tissue stretcher 10 (e.g., removing themechanical coupling between the locking members 56 and the first andsecond actuators 32 a,b,c,d, 34 a,b,c,d) can allow the respective stressloads to be applied to tissue to be biaxially stretched.

The planar surface of the base 52 can be about as wide and as long asthe overall length of the first and second guides 12 a,b, 16 a,b suchthat the first and second guides 12 a,b, 16 a,b can remain supported onthe base 52 to provide stability while an operator mounts tissue on thefirst mounts 26 a,b and the second mounts 28 a,b. In some embodiments,the base 52 is a substantially rigid material that can be easily cleanedand/or sterilized and resists corrosion over time. For example, the base52 can be poly(methyl methacrylate) (PMMA).

The support 54 is mechanically couplable to the aperture 22 defined bythe first and second guides 12 a,b, 16 a,b such that the first andsecond guides 12 a,b, 16 a,b are stationary relative to the base 52. Forexample, the support 54 can be complementary to the aperture 22 of thetissue stretcher 10 such that the support 54 is movable through theaperture 22 to hold the tissue stretcher 10 fixed in place along thesubstantially planar surface of the base 52. The support 54 can bedimensioned relative to the aperture 22 such that the support 54 holdsthe tissue stretcher 10 in place but is removable from the aperture 22of the tissue stretcher 10 by the application of manual force by anoperator.

Referring now to FIGS. 4-6, certain methods of biaxially stretchingtissue using the tissue stretcher 10 and the jig 50 are described. Thetissue stretcher 10 is mounted to the jig 50 by positioning the support54 of the jig 50 through the aperture 22 defined by the tissue stretcher10. This positioning allows the tissue stretcher 10 to be positionedrelative to the locking members 56 extending from the base 52. Usinggripping portions 30 of the respective first and second mounts 26 a,b,28 a,b, an operator can push each of the first and second mounts 26 a,b,28 a,b toward one another along the respective first and second guides12 a,b, 16 a,b such that the first and second mounts 26 a,b, 28 a,b aredisposed within the area circumscribed by the locking members 56. Thishas the effect of compressing the respective first and second actuators32 a,b,c,d, 34 a,b,c,d such that respective first and second stressloads are set.

The tissue 70 is mounted over hooks 40 extending from the respectivefirst and second mounts 26 a,b, 28 a,b such that a substantially planarsurface of the tissue 70 is parallel to a plane along which the firstand second actuators 32 a,b,c,d, 34 a,b,c,d move. The tissue 70 can bebovine pericardium, equine pericardium, or porcine pericardium. In someembodiments, the tissue 70 is a patch cut from a pericardial sac. Incertain embodiments, the tissue 70 has an initial thickness of about 0.1mm to about 0.7 mm.

With the tissue 70 mounted on the first and second mounts 26 a,b, 28a,b, the jig 50 can be moved relative to the tissue stretcher 10 suchthat the support 54 and the locking members 56 are decoupled from thetissue stretcher 10. For example, the jig 50 can be moved relative tothe tissue stretcher 10 such that the support 54 and the locking membersare substantially simultaneously decoupled from the tissue stretcher.

The decoupling of the jig 50 from the tissue stretcher 10 allows thefirst actuators 32 a,b,c,d to move the first mounts 26 a,b away from oneanother along the first axis 14 to apply a first set stress load to thetissue 70. Similarly, the decoupling of the jig 50 from the tissuestretcher 10 allows the second actuators 34 a,b,c,d to move the secondmounds 28 a,b away from one another along the second axis 18 to apply asecond set stress load to the tissue 70. In some embodiments, the firstmounts 26 a,b and the second mounts 28 a,b move away from each othersubstantially simultaneously such that the first and second set stressloads and, thus, first and second stresses are applied to the tissue 70substantially simultaneously. Additionally or alternatively, the firstand second set stress loads applied to the tissue 70 through therelative movement of the first and second mounts 26 a,b, 28 a,b suchthat the resulting stresses applied to the tissue 70 can be less thanthe elastic limit of the tissue 70. In some embodiments, the first andsecond stress set stress loads is about 0.1 N to about 2 N. In certainembodiments, the first and second set stress applied to the tissue 70 isabout 0.01 N/mm² to about 2 N/mm².

In some embodiments, the first and second set stress loads are appliedto the tissue 70 by the tissue stretcher 10 for about 30 minutes toabout 120 minutes (e.g., in a glutaraldehyde solution). In certainembodiments, the tissue can be removed from the tissue stretcher 10 andexposed to a glutaraldehyde solution for about one day to about twoweeks. Additionally or alternatively, the tissue 70 can be mounted onthe tissue stretcher 10 and exposed to a non-cross linking-solution(e.g., phosphate-buffered saline or saline) for about 30 minutes toabout 120 minutes prior to exposure to a glutaraldehyde solution forabout one day to about two weeks. The exposure of the tissue 70 to thenon-cross linking solution could be carried out between about 4° C. toabout 37° C. (e.g., about 20° C.). The exposure of the tissue 70 to thenon-cross-linking solution, while the tissue 70 is being biaxiallystressed, can allow the tissue 70 to respond to the stress and reorientprior to locking that structure in place with a cross-linking solutionsuch as glutaraldehyde.

As the tissue stretcher 10 acts on the tissue 70 to apply the first andsecond set stress loads, the tissue stretcher 10 and the tissue 70 canbe exposed to a glutaraldehyde solution 80. This exposure can range induration from about 10 minutes to about 3 hours (e.g., about 30 minutesto about 120 minutes). Such exposure of the tissue 70 to theglutaraldehyde solution 80 can facilitate crosslinking of the tissue 70such that the tissue 70 will hold the stretched position after the firstand second stress loads are removed (e.g., after the tissue 70 isremoved from the tissue stretcher 10). In some embodiments, the tissue70 is held in the stretched position such that the average thickness ofthe tissue 70 is reduced. For example, the average thickness of thetissue 70 held in the stretched position can be about 0.1 mm to about0.4 mm. In some implementations, biaxial stretching of tissue and fixingthe tissue results in little to no increase in thickness in the tissue.In these embodiments, as compared to fixing tissue under uniaxial stressloading or no stress loading, biaxial stretching can result in thinnerfixed tissue. In implementations in which the tissue 70 is part of anintraluminally delivered prosthetic heart valve, such a reduction in thethickness of the tissue 70 can, for example, reduce the sheathing forcesassociated with the prosthetic heart valve and/or reduce the overallprofile of the prosthetic heart valve for easier delivery to theimplantation site.

While certain embodiments have been described, other embodiments arepossible.

For example, while the first and second mounts 26 a,b and 28 a,b havebeen described as being movable relative to the respective first andsecond guides 12 a,b, 16 a,b through low-friction slidable contact,other embodiments are additionally or alternatively possible. In someembodiments, referring to FIG. 7, a mount 26 a′ includes bearings 42 a,bthat can form at least a portion of the moving interface between themount 26 a′ and the first guides 12 a,b. For example, the bearings 42a,b can be polyoxymethylene or polytetrafluoroethylene such that thebearings 42 a,b are grease-free to reduce the likelihood ofcontamination of the tissue 70. Additionally or alternatively, eachbearing 42 a,b can be a flange bearing to facilitate placement of thebearings 42 a,b on the mount 26 a′.

As another example, while the thickness of the tissue 70 has beendescribed as being reduced through the application of set biaxial stressloads, other embodiments are additionally or alternatively possible. Insome embodiments, referring to FIGS. 8-9, a tissue modification system90 includes a downdraft table 98, clamps 92, and a planar shaver 94. Theclamps 92 can secure the tissue 70 (e.g., a substantially planar patchof pericardial tissue) to the downdraft table 98 such that a relativelysmooth substantially planar surface of the tissue 70 is disposed towardthe table 98 while a relatively rough substantially planar surface ofthe tissue 70 is disposed toward the planar shaver 94. Vacuum pressureis applied through vents 100 defined by the downdraft table 98. Thevacuum pressure draws the tissue 70 toward the substantially planarsurface of the table 98 while the planar shaver 94 moves over the tissue70, for example, in the direction 95 indicated in FIG. 8 such that ablade 96 of the planar shaver 94 removes tissue along at least a portionof the relatively rough substantially planar surface of the tissue 70.Such movement of the planar shaver 94 over the tissue 70 can result in aglobal reduction of the thickness of the tissue 70. Saline solution canbe applied to the tissue 70 to keep the tissue 70 moist throughout theprocess of mounting the tissue 70 to the table 98 and moving the planarshaver 94 over the tissue 70.

In certain embodiments, referring to FIG. 10, a tissue modificationsystem 102 includes a laser 112 (e.g., a femtosecond laser) proximate tothe table 98 to direct a laser beam across at least a portion of aleaflet 104 clamped by clamps 92 to the table 98. For example, theleaflet 104 can be a leaflet of a prosthetic heart valve, and theleaflet can include a coaptation portion 106 substantially opposite anarcuate edge 105 having a first end 107 and a second end 109. Theleaflet 104 can also include a belly 108 extending from the arcuate edge105 to an axis 110 defined by the first and second ends 107, 109 of thearcuate edge 105.

The laser 112 can, for example, direct the laser beam across the leaflet104 to remove tissue locally along a portion of the leaflet 104 toachieve a thickness profile in which portions of the leaflet 104 may bethinner than portions of the leaflet 104. For example, the laser 112 candirect the laser beam across the leaflet 104 to remove tissue from thebelly 108 of the leaflet 104. Such local removal of tissue from theleaflet 104 can, for example, reduce the forces associated withsheathing a prosthetic heart valve including the leaflet 104 as comparedto a leaflet without material removed from its respective belly.

As still another example, while tissue stretching and tissue removalhave been described for reducing the forces associated with sheathing aprosthetic heart valve including one or more tissue leaflets, otherembodiments are additionally or alternatively possible. For example,FIGS. 11A-11D illustrate a prosthetic heart valve 112 that includestissue 114 (e.g., one or more leaflets movable between open and closedpositions to permit and restrict, respectively, the flow of bloodthrough the heart) and a sheath 120 that can be used for intraluminaldelivery of the prosthetic heart valve 112. The prosthetic heart valve112 can be stored initially in a first solution 116 that is isotonicwith the tissue 114. For example, the prosthetic heart valve 112 can beshipped in the first solution 116. The prosthetic heart valve 112 can beexposed to a second solution 118 (e.g, in the operating room just priorto implantation of the prosthetic heart valve 112) such that water inthe tissue 114 flows out of the tissue 114 to reduce at least onedimension of the tissue 114.

For example, the second solution 118 can be a solution hypertonic to thetissue 114. In some embodiments, the second solution 118 is about 75percent to about 80 percent water. Additionally or alternatively, thetonicity (e.g., hypertonicity) of the second solution 118 is about 339mOsm/L (110% of isotonic, ˜10 g/L) to about the limit of solubility ofNaCl in water, 12.3 Osm/L (40× isotonic, 359 g/L).

Additionally or alternatively, the second solution 118 can includealcohol. For example, the exposure of the tissue 114 to the secondsolution 118 can include exposure of the tissue 114 to a series ofgraded alcohol solutions (e.g., about 50%, about 60%, about 70%, about90%, about 95%, about 100%) to remove water and thin the tissue 114prior to loading the tissue. Additionally or alternatively, exposure ofthe tissue 114 to the second solution 118 can include exposures of thetissue 114 to a series of graded ethylene glycol solutions (e.g., about50%, about 60%, about 70%, about 90%, about 95%, about 100%) to removewater and thin the tissue 114 prior to loading. In some embodiments, thetissue 114 is exposed to alcohol or ethylene glycol solutions prior tostress loading the tissue 114.

In some embodiments, the tissue 114 can be exposed to the secondsolution 118 as an acute rinse (e.g., for about 30 seconds to about 15minutes). In certain embodiments, the tissue 114 can be stored in thesecond solution 118 to eliminate, for example, the need for a rinse. Thefirst solution 116 and the second solution 118 can be at substantiallythe same temperature (e.g., room temperature) which, as compared totechniques that require exposing tissue of a prosthetic heart valve tosolutions at different temperatures, can reduce the need for theend-user to maintain a controlled difference in temperature to achieve adesired reduction in the size of tissue of a prosthetic heart valve.

In some embodiments, removal of water from the tissue 114 throughexposure to the second solution 118 is done immediately prior tosheathing the valve 112. In certain embodiments, removal of water fromthe tissue 114 through exposure to the second solution 118 is done sometime prior to sheathing the valve 112. For example, the tissue 114 canbe exposed to the second solution 118 during the final manufacturing ofthe valve 112 that includes the tissue 114. Through exposure of thetissue 114 to the second solution 118 during the final manufacturing ofthe valve 112, the tissue 114 can, for example, undergo terminalsterilization in a dehydrated state. As compared to tissue 114 in ahydrated state, the tissue 114 in a dehydrated state can betterwithstand exposure to forms of terminal sterilization that tend todamage and/or alter tissue 114 in a hydrated state. For example,terminal sterilization of the tissue 114 in the dehydrated state caninclude exposure of the dehydrated tissue to electron beam (e-beam)and/or ethylene oxide (EtO). Additionally or alternatively, with thetissue 114 in the dehydrated state, the valve 112 could be sheathed orunsheathed during sterilization and storage, and the entire assembly(including the valve 112, a delivery system, and a handle) could besterilized in one package, without the need for storing the valve inliquid.

With the size of the tissue 114 reduced through exposure to the secondsolution 118, the sheath 120 can be advanced distally to sheath theprosthetic heart valve 112 for intraluminal delivery to an implantationsite in a patient. As compared to tissue that has not been exposed tothe second solution 118, the force required to sheath the tissue 114will be reduced. Upon implantation and exposure to blood in the patient,the tissue 114 will absorb water to return to its original size.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the first and second guides can each be a single rod or threeor more rods. Accordingly, other embodiments are within the scope of thefollowing claims.

What is claimed is:
 1. A tissue modification apparatus comprising: afirst guide defining a first axis and a second guide defining a secondaxis intersecting the first axis; a first set of mounts and a second setof mounts, the first set of mounts movable relative to one another alongthe first axis, the second set of mounts movable relative to one anotheralong the second axis; and a first actuator and a second actuator eachsettable to a stress load, the first actuator and the second actuatormovable, respectively, along the first and second axis to transmit eachrespective set stress load to a piece of tissue mechanically coupled tothe first and second set of mounts.
 2. The tissue modification apparatusof claim 1 wherein the second axis is substantially perpendicular to thefirst axis.
 3. The tissue modification apparatus of claim 1 wherein thesecond axis intersects the first axis in a plane substantially parallelto a substantially planar surface of the piece of tissue mechanicallycoupled to the first and second set of mounts.
 4. The tissuemodification apparatus of claim 1 wherein at least a portion of thefirst guide is mechanically coupled to at least a portion of the secondguide.
 5. The tissue modification apparatus of claim 1 furthercomprising a plurality of first guides substantially parallel to orcoaxial with the first axis and a plurality of second guidessubstantially parallel to or coaxial with the second axis.
 6. The tissuemodification apparatus of claim 1 wherein the first guide and the secondguide each comprise at least one passivated surface.
 7. The tissuemodification apparatus of claim 1 wherein the first guide and the secondguide each comprise at least one polished surface.
 8. The tissuemodification apparatus of claim 1 wherein the first guide and the secondguide each include at least one collar stationary relative to therespective first and second guide, the at least one collar disposedalong the respective first and second axes, and the at least one collarlimiting movement of the respective first and second set of mounts alongthe respective first and second axes.
 9. The tissue modificationapparatus of claim 1 wherein each mount of the first set of mounts islarger than each of the second set of mounts along axes perpendicular tothe respective first and second axes.
 10. The tissue modificationapparatus of claim 1 wherein each mount of the first and second set ofmounts comprises mitered end portions.
 11. The tissue modificationapparatus of claim 10 wherein the mitered end portions of each mount ofthe first set of mounts is complementary to the mitered end portions ofeach mount of the second set of mounts.
 12. The tissue modificationapparatus of claim 1 wherein the first and second actuators are eachmovable along the respective first and second axes to transmit eachrespective set stress load substantially simultaneously to a piece oftissue.
 13. The tissue modification apparatus of claim 1 wherein thefirst and second actuators each comprise at least one spring.
 14. Thetissue modification apparatus of claim 1 wherein each mount of the firstand second set of mounts comprises at least one surface in slidablecontact with the respective first and second guides along the respectivefirst and second axes.
 15. The tissue modification apparatus of claim 1wherein each mount of the first and second set of mounts comprises abearing in rolling contact with a respective first and second guide. 16.The tissue modification apparatus of claim 1 wherein each mount of thefirst and second set of mounts comprises a plurality of hooks extendingaway from each respective mount.
 17. A tissue modification systemcomprising: a tissue stretcher comprising a first guide defining a firstaxis and a second guide defining a second axis intersecting the firstaxis; a first set of mounts and a second set of mounts, the first set ofmounts movable relative to one another along the first axis, the secondset of mounts movable relative to one another along the second axis; anda first actuator and a second actuator mechanically coupled,respectively, to the first and second set of mounts; and a jigcomprising a base, a support extending from the base, the first guideand/or the second guide mechanically couplable to the base such that thefirst and second guides are stationary relative to the base, and aplurality of locking members mechanically couplable to the base andmovable with respect to the first and second actuators to allow movementof the first and second actuators along the respective first and secondaxes to transmit a respective stress load to a piece of tissuemechanically coupled to the first and second set of mounts.
 18. Thetissue modification system of claim 17 wherein the first and secondactuators are movable relative to the base to set respective stressloads and the plurality of locking members are movable to hold the firstand second actuators at the respective set stress loads.
 19. The tissuemodification system of claim 17 wherein the support and the lockingmembers are simultaneously mechanically decouplable from the tissuestretcher.
 20. A tissue modification method comprising: moving a firstpair of mounts toward one another along a first axis to set a firstload; moving a second pair of mounts toward one another along a secondaxis, intersecting the first axis, to set a second load; mechanicallycoupling a substantially planar sheet of tissue to the first pair ofmounts and to the second pair of mounts; moving the first pair of mountsaway from one another along the first axis to apply the set first loadto the substantially planar sheet of tissue; and moving the second pairof mounts away from one another along the second axis to apply the setsecond load to the substantially planar sheet of tissue.